Green Rice: Resource-conserving rice production
Reduced water use in rice production typically leads to increased N2O emissions. Understanding where and when N2O is being produced in the soil profile can help to advance our knowledge of nitrogen cycling and N2O emissions in relation to water management.
Background (completed research project)
Previous research has evaluated surface emissions of N2O as a function of water management, but few studies have investigated N2O production and consumption at depth. The implementation of drainage events does affect N2O emissions but we need to elucidate the sources of emissions.
The three main aims of this research were to:
- evaluate how water management affects N2O production and consumption across the soil profile and ultimately surface emissions from alternative rice systems;
- advance methods for the interpretation of in-situ natural abundance N2O isotopes;
- evaluate iron reduction and iron–nitrogen redox reactions and NH4+ fixation as a function of water management in rice systems.
N2O emissions increased with the implementation of alternate wetting and drying (AWD) irrigation relative to conventional flooding. Production and emission of N2O was highest early in the growing season and was linked to NH4+ availability in the upper soil layers. Subsurface N2O production and surface emissions were mostly unaffected by urea fertilization and were more related to the mineralization of native soil N or pre-season fertilizer application. Our isotope results show that most N2O originated from denitrification or nitrifier-denitrification. High rates of N2O reduction were observed in all treatments, regardless of water management. Estimated N2 emissions ranged from 245-333 g N ha-1 d-1.
We observed a high potential for NH4+ fixation in all treatments. Total fixation increased with anaerobic conditions, while fixation-release dynamics were highest in the AWD treatments. It appears that iron-nitrogen coupled redox reactions increased in the water-seeded treatments and may have contributed to fertilizer NH4+ loss in these systems.
Importance for research
There is a real potential to use N2O isotope signatures to source-partition N2O emissions and estimate N2 emissions.
The relatively large and dynamic concentrations of fixed NH4+ control pore water NH4+ availability and future studies should consider this as dynamic rather than static pool of NH4+.
The implementation of water saving practices tended to increase N2O emissions, most significantly in a dry seeded AWD treatment, relative to a water seeded AWD treatment or the conventional practice of water seeded + flooding. Management to reduce nitrification such as delaying pre-season fertilizer application or the use of slow release fertilizers may help to reduce N losses. Future work should investigate if processes such as NH4+ fixation can be beneficially manipulated to further regulate NH4+ availability in relation to plant demand, thereby mitigating N2O emissions and improving the nitrogen use efficiency of cropping.
Greenhouse gas emissions from paddy rice soils under alternative irrigation management